Being neighbourly

A long time ago I looked after a friend /neighbours baby one day of the first week she tried a job. Her family did the other days. At the end of the week she told me she was taking it permanently. And proceeded to tell me what days I would be designated. Lol! I was a SAH mum with 3 children. I said right away no I couldn't do any of it. I had only intended to help out one day. We were still friends. You just have to speak up. Not easy but needs must. Same with gc. Its easy to say sorry I haven't the energy to do so much. It is the truth and family should understand.

I think you have to be quick - tell her how dreadful you feel about it, but that you really don't feel able to do as much as you have previously.
When we had to tell our son that we couldn't provide the childcare for his child as we had previously (for his sibling's children), we used the words 'we don't feel safe, for that amount of time'.
I'm not sure why this has been left so late, which makes finding alternative care very difficult: if it is the neighbour making assumptions, then I think you are on safe ground - if you knew about this assumption and just let it drift then you do need to apologise.
Certainly talk up the arthritis - that you don't move as quickly as you used to etc. You can give the impression that is has recently got somewhat worse, leaving you less able.
The other thing I would think about - you don't say the ages of the children - how much can they help you? We insist on our DGCs setting the table and clearing away etc. Also, could they bring packed lunches? I don't know if this would help, or if it is the general responsibility that you are concerned about.
I do think that an honest conversation, recognising the affection you feel for the neighbour and the child, and an offer to do a small amount to help, will be useful.

You do have to learn to stand up for yourself firmly but nicely.
I was horrified to see someone we know who is a tiny, bent 85 year woman, pushing a rather large (probably age 3) great-grandchild in a pushchair up a hill yesterday. We couldn't stop, we were in a stream of traffic turning the other way.

I just could not understand why family would put on her like that.

Think you have to be up front sluttygran and say you are unable to provide child care as you have committments to your family. It's no good beating about the bush as this leads to misunderstanding.

Hilda’s response is pretty good. Also perhaps add that dr has suggested you take it a bit easier. Could you cut out one day of dgc. Not easy to say.

Yes, good idea HildaW

Well, you need to get in quick with....a cheery smile on your face saying something like. 'Gosh its going to be really hard work looking after my grandchildren this year, I do love doing it but as I get older (and lay on a few worrying symptoms) I find myself coping less well'....then take a deep breath and ask her....'and who is looking after little **?' ....and let it just sit there for a minute. Smile the whole time but let her know you have hit your limit. Good luck.